I. Field of the Invention
The present invention relates generally to mold blanks for manufacturing plastic eyeglass lenses and, more particularly, to such a mold blank having a flat top bifocal segment.
II. Description of the Prior Art
In the manufacture of plastic eyeglass lenses, an optical surface is first machined onto one side of a platform. Such platforms are typically constructed of ceramic.
A glass mold blank is placed onto the platform optical surface and the assembly is then placed into a furnace. The furnace is heated to a preselected temperature at which the mold blank thermally deforms and is sagged against the optical surface of the platform so that the mold blank conforms to the optical surface. Upon cooling, the mold blank is used to manufacture plastic eyeglass lenses.
Thermal sagging has not been previously used to construct a mold blank having a flat top bifocal segment.